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<div class="titlepage"><div><div><h3 class="title">
<a name="phoenix.starter_kit.lazy_functions"></a><a class="link" href="lazy_functions.html" title="Lazy Functions">Lazy Functions</a>
</h3></div></div></div>
<p>
As you write more lambda functions, you'll notice certain patterns that you
wish to refactor as reusable functions. When you reach that point, you'll
wish that ordinary functions can co-exist with phoenix functions. Unfortunately,
the <span class="emphasis"><em>immediate</em></span> nature of plain C++ functions make them
incompatible.
</p>
<p>
Lazy functions are your friends. The library provides a facility to make
lazy functions. The code below is a rewrite of the <code class="computeroutput"><span class="identifier">is_odd</span></code>
function using the facility:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">is_odd_impl</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">bool</span> <span class="identifier">result_type</span><span class="special">;</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></span>
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Arg</span> <span class="identifier">arg1</span><span class="special">)</span> <span class="keyword">const</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="identifier">function</span><span class="special"><</span><span class="identifier">is_odd_impl</span><span class="special">></span> <span class="identifier">is_odd</span><span class="special">;</span>
</pre>
<h5>
<a name="phoenix.starter_kit.lazy_functions.h0"></a>
<span class="phrase"><a name="phoenix.starter_kit.lazy_functions.things_to_note_"></a></span><a class="link" href="lazy_functions.html#phoenix.starter_kit.lazy_functions.things_to_note_">Things
to note:</a>
</h5>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Result type deduction is implemented with the help of the result_of protocol.
For more information see <a href="http://www.boost.org/doc/libs/release/libs/utility/utility.htm#result_of" target="_top">Boost.Result
Of</a>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">is_odd_impl</span></code> implements
the function.
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">is_odd</span></code>, an instance of
<code class="computeroutput"><span class="identifier">function</span><span class="special"><</span><span class="identifier">is_odd_impl</span><span class="special">></span></code>,
is the lazy function.
</li>
</ul></div>
<p>
Now, <code class="computeroutput"><span class="identifier">is_odd</span></code> is a truly lazy
function that we can use in conjunction with the rest of phoenix. Example:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">is_odd</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">));</span>
</pre>
<p>
(See <a href="../../../../example/function.cpp" target="_top">function.cpp</a>)
</p>
<h5>
<a name="phoenix.starter_kit.lazy_functions.h1"></a>
<span class="phrase"><a name="phoenix.starter_kit.lazy_functions.predefined_lazy_functions"></a></span><a class="link" href="lazy_functions.html#phoenix.starter_kit.lazy_functions.predefined_lazy_functions">Predefined
Lazy Functions</a>
</h5>
<p>
The library is chock full of STL savvy, predefined lazy functions covering
the whole of the STL containers, iterators and algorithms. For example, there
are lazy versions of container related operations such as assign, at, back,
begin, pop_back, pop_front, push_back, push_front, etc. (See <a class="link" href="../modules/stl.html" title="STL">STL</a>).
</p>
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